Fluorescent lights are well known in the lighting industry as efficient light sources. Fluorescent lights have a wide variety of domestic and industrial applications, including lighting rooms, lighting workspaces and lighting signs, for example. In general, fluorescent lights comprise one or more fluorescent tubes, each tube providing a separate light source. Fluorescent tubes can vary in size, with larger tubes generally drawing more power and providing more light.
As is well known in the art, fluorescent tubes are a gas discharge type of light source. A typical prior art fluorescent tube 10 is shown schematically in FIG. 1, along with its ballast 12, its power supply 14 and its starter switch 20. Ballast 12 conventionally comprises at least one ferromagnetic inductor 13. Fluorescent tube 10 comprises a pair of filaments 16, 18 which typically have some slight resistance on the order of approximately 0.5–30 Ω. Tube 10 also contains a small amount of mercury (initially a liquid) and one or more inert gases, such as argon, which are under low pressure.
Lighting tube 10 involves creating current flow or “arc” through tube 10 between filaments 16, 18. In fluorescent tubes commonly referred to as the “hot cathode” type, creating the current in tube 10 typically involves preheating at least one of filaments 16, 18 to cause thermionic emission of electrons. Filaments 16, 18 may be coated with various types of materials well known in the art to increase the amount of thermionic emission. Preheating filaments 16, 18 may be said to “boil off” electrons. In addition to preheating filaments 16, 18, creating a current arc through tube 10 also typically involves providing a relatively large “ignition voltage” across tube 10. The ignition voltage induces ionization of the inert gas in tube 10 and ignites the current flow between filaments 16, 18. The required ignition voltage for a given tube 10 varies depending on many factors. Typical commercial fluorescent tubes of the hot cathode type operate with an ignition voltage in a range between 300–800 V AC RMS. The thermionic emission of electrons into tube 10 during preheating of filaments 16, 18 tends to reduce the required ignition voltage. Typically, the ignition voltage is provided between filaments 16, 18 by ballast 12, which works together with starter switch 20 as explained briefly below.
During preheating, starter switch 20 is closed and AC current flows through inductive ballast 12, filament 16, switch 20 and filament 18. This current preheats filaments 16, 18, resulting in thermionic emission of electrons, and also builds up a magnetic field in the inductor 13 of ballast 12. During preheating, there may also be some ionization of the gas in tube 10; however, the voltage across tube 10 (i.e. between filaments 16, 18) is not sufficient to create a current arc through the gas in tube 10. Consequently, almost all of the current flows through starter switch 20 and correspondingly little or no current flows through tube 10.
When a sufficient number of electrons have been thermionically emitted from filaments 16, 18 and sufficient magnetic field has been established in inductor 13 of ballast 12, starter switch 20 is opened, briefly cutting off current flow through ballast 12. When the current is cut off from ballast 12, the magnetic field in the inductor 13 of ballast 12 collapses, causing an inductive voltage spike. This inductive voltage spike provides the ignition voltage across tube 10 (i.e. between filaments 16, 18), which in turn ionizes the gas in tube 10 and creates an arc of current between filaments 16, 18.
Instead of flowing through starter switch 20, current now flows through tube 10. Current flow is maintained through tube 10 by electrons emitted from hot filaments 16, 18 and by the electrons and ionized gas particles in tube 10. Filaments 16, 18 remain hot because of the emission of electrons. These moving ions and electrons provide energy to the mercury contained in tube 10, converting some of the mercury from liquid to gas. Collisions between electrons and gaseous mercury atoms cause electrons in the gaseous mercury atoms to occupy higher energy states. When these mercury electrons return to their ground energy states, they release ultra-violet photons. Tube 10 is typically coated with phosphors (not shown), which absorb the ultraviolet photons. Absorption of ultraviolet photons causes the electrons of the phosphors to occupy higher energy states. When these phosphor electrons return to their ground energy states, they release photons in the visible spectrum.
When the arc is created through tube 10, the resistance between filament 16 and filament 18 decreases. More specifically, the flow of electrons and ions through tube 10 creates collisions with other atoms, liberating more ions and electrons and facilitating the flow of more current. Inductive ballast 12 prevents damage to filaments 16, 18 and tube 10 by limiting the total current through tube 10. Since power supply 14 typically provides a known AC signal, the inductance of inductor 13 of ballast 12 may be selected appropriately to limit the current through tube 10 to a desired level.
More recently designed fluorescent tubes, known as “rapid start” tubes, incorporate the same basic principles as the classical tubes described above. Other modern fluorescent tubes, known as “instant start” tubes, eliminate the preheating stage and ignite current flow through the tube with a corona discharge. The corona discharge associated with instant start tubes causes stress on tube components, particularly the filaments, and reduces the service life of the tube. Still other types of fluorescent tubes, known as “cold cathode” tubes incorporate relatively large, typically iron, electrodes. Cold cathode tubes require extremely large voltage drops between their electrodes to generate electrons through the impact of accelerated ions, which is referred to as “secondary emission”. These large voltages are a safety concern, particularly in multi-tube applications. Modern fluorescent tubes may also utilize more complex solid state electronic ballasts, which use high frequency switching techniques to provide ignition voltage and current regulation in a manner similar to classical inductive ballasts.
For some applications, such as industrial signage for example, there remains a general need for low cost control systems capable of independently controlling a plurality of fluorescent tubes. Because the ignition voltages of fluorescent tubes can be quite high, it is desirable to generate such voltages in close proximity to the tubes, and to thereby minimize the required voltage(s) on exposed wiring connections.